In the linear optical material, for example, a lens, a prism and the like, the light beam only passes through the optical material and does not change a certain property of the optical material itself. The effect in which the refractivity of the material is changed by the light is referred to as the photorefractive effect in short. However, the photorefractive effect particularly refers to the effect in which a charge field is formed by the photoelectric effect when the optical material is radiated by the light beam and then the refractivity of the optical material varies with the distribution of the light intensity in space due to the photoelectric effect rather than refers to all the changes of the refractivity induced by the light beam. Since the optical property of the photorefractive crystal can be changed by the light beam passing through the optical material, and thus such optical material belongs to non-linear optical material. Similar to the film photosensitizer used in the usual photography, the photorefractive crystal can record an intricate pattern of the light; further, the photorefractive crystal has more advantages over the film in performance, such as large capacity, real-time property, recyclable property, easy to be preserved and the like; for example, an image of the light beam recorded in the photorefractive crystal can be erased, while the film only records an image once being exposed and the image recorded can not be erased; further, for instance, the photorefractive crystal can store 5,000 different images in a volume of 3 cm3, and can display any one of the images quickly.
Holography refers to all the information of the light wave, i.e., the amplitude information and the phase information of the light wave. The normal photography only records the intensity information of the light wave (i.e. the amplitude information) but loses the phase information of the light wave. Since the image reproduced by the holography has a strong stereoscopic sense, the three-dimensional (3D) display based on the holography has gained more and more attention. The holographic 3D display technique mainly comprises the synthetic holography 3D display, the digital holography 3D display and the holographic 3D display based on the photorefractive crystal. The holographic 3D display technique based on the photorefractive crystal utilizes the property of the photorefractive effect of the photorefractive crystal, and all the information (the amplitude information and the phase information) of the object light wave is stored in the recordable medium when it is recorded according to the interference principle; when the recordable medium is radiated by the reproducing light wave, the original object light wave is reproduced according to the diffraction principle, so that an vivid stereoscopic image is reproduced.
In the 3D display technique based on the photorefractive crystal in the prior art, when the information is recorded in holography, the coherent reference light beam and object light beam are incident to the photorefractive crystal in different angles, and interference fringes are generated at the portions where the reference light beam and the object light beam intersect; the information is recorded in the photorefractive crystal by means of the photorefractive effect, and a holographic image is formed. When it is reproduced in holography, the incident angle, the frequency and the optical path of the reference light beam are maintained unchanged, and the reference light beam is incident to the photorefractive crystal having the holographic image recorded therein as a reproducing light beam, so that an observer located in a certain orientation (which guarantees that the observer is roughly located in the angle at which the object light beam is incident to the photorefractive crystal when the holographic image is formed) views an image recorded in the photorefractive crystal; then the photorefractive crystal is rotated and the reproducing light beam is maintained unchanged, so that the observer views another holographic image recorded in the photorefractive crystal.